ADU (Accessory Dwelling Unit) Cost Calculator

In 2026, building an accessory dwelling unit (ADU) typically costs $150 to $350+ per square foot, so total costs commonly range from around $80,000 for a small garage conversion to $300,000+ for a larger detached new-build unit — with most ADUs landing somewhere between roughly $100,000 and $250,000. The cost depends mainly on the ADU type (a garage conversion is the most economical since the structure exists; a basement/interior conversion is also cheaper; an attached addition costs more; and a detached new-build is the most expensive as a complete new structure), the size (priced per square foot, with ADUs ranging from ~400 to ~1,200 sq ft), the finish level (basic, standard, or high-end), and the kitchen/bath configuration (a full ADU with a complete kitchen and bath costs more than an in-law suite without a full kitchen). Because an ADU is essentially a small complete home, it includes a kitchen, bathroom, living space, and all the systems (plumbing, electrical, HVAC), which is why the per-square-foot cost can be higher than a simple room addition. Major additional costs include design/architecture, engineering, and permits (significant for ADUs), separate or upgraded utilities (electrical service, water/sewer connections, possibly separate meters), a dedicated HVAC system, trenching utilities out to a detached unit, kitchen appliances, and matching the main house's exterior. Site conditions, local labor rates, and especially local regulations and permit/impact fees heavily influence the cost. This calculator lets you set the size, ADU type, finish, and kitchen/bath config to estimate your project. Pricing varies widely by region (ADUs cost much more in high-cost areas), the type and size, the site, and the contractor. Despite the cost, ADUs add living space, rental income potential, and property value.

An ADU (accessory dwelling unit) is a secondary, self-contained living unit on the same lot as a primary home — it's a complete small dwelling with its own kitchen, bathroom, sleeping/living area, and entrance, and goes by many names: granny flat, in-law suite, backyard cottage, casita, secondary suite, guest house, or carriage house. ADUs are used for housing family members (aging parents, adult children), generating rental income, hosting guests, a home office/studio, or flexible space, and they've become popular as many areas have relaxed regulations to encourage them (for affordable housing and density). The main types: A detached ADU is a separate standalone structure in the yard (a backyard cottage/casita) — the most flexible and private, but the most expensive since it's a complete new building with its own foundation, walls, and roof. An attached ADU is built as an addition connected to the primary house (sharing a wall) — it's an addition with its own entrance and facilities. A garage conversion ADU transforms an existing garage (attached or detached) into living space — the most economical type because the structure (foundation, walls, roof) already exists, so you're mainly finishing the interior and adding the kitchen/bath/systems. A basement or interior conversion ADU converts a basement, attic, or part of the existing home into a separate unit with its own entrance and facilities — also economical since it uses existing space. A junior ADU (JADU) is a smaller unit (often a converted bedroom) within the existing home, typically with a kitchenette and sometimes a shared bath, subject to specific rules. The right type depends on your property (lot size, existing garage/basement), budget, privacy needs, and local regulations (which dictate what's allowed, sizes, setbacks, and parking). This calculator lets you choose garage conversion, basement/interior conversion, attached, or detached, which significantly affects the cost. An ADU's defining feature is that it's an independent living unit (with its own kitchen and bath) on the property. Local rules vary, so check what types and sizes your jurisdiction permits.

The terms overlap and are often used interchangeably, but there's a meaningful distinction centered on whether the space is a fully independent dwelling — particularly whether it has its own kitchen. A true ADU (accessory dwelling unit) is a complete, independent living unit: it has its own kitchen, bathroom, sleeping/living area, and separate entrance, so someone can live there fully independently (cook, bathe, sleep) — it's essentially a small separate home, and it can typically be legally rented out as a separate dwelling (where permitted). The defining feature is the full kitchen and independent facilities. An in-law suite (or in-law apartment) is a living space designed for a family member (like aging parents) that includes a bedroom and usually a private bathroom, and often a sitting area and separate entrance — but it may or may not have a full kitchen. If an in-law suite has a full kitchen and meets the requirements, it essentially is an ADU; if it lacks a full kitchen (perhaps just a bedroom and bath, or a kitchenette), it's more of a suite attached to the main home rather than a fully independent unit, and may be treated differently by code (it might not qualify as a separate dwelling unit). The kitchen is often the key legal/functional dividing line: a full kitchen generally makes it a separate dwelling unit (ADU), while no full kitchen makes it an accessory living space/suite within the home. Practically: an ADU is for independent living or rental (with full facilities); an in-law suite is for housing a relative comfortably (private space, possibly without a full kitchen so they share some of the main home, or a kitchenette for partial independence). The choice affects cost (a full kitchen adds cost), the permitting/legal status, and the use (rentability). This calculator includes kitchen/bath options — full kitchen + bath (true ADU), kitchenette + bath, or an in-law suite with no full kitchen — reflecting these differences in scope and cost. Decide based on the intended use (independent dwelling/rental vs. housing a family member) and your local rules on what constitutes a separate dwelling unit. Terminology and legal definitions vary by area.

Yes — converting an existing garage into an ADU is generally the most economical type of ADU, because much of the structure already exists, so you avoid the cost of building a new foundation, walls, and roof from scratch. Why it's cheaper: a garage already has a foundation/slab, framing, walls, and a roof, so a garage conversion mainly involves finishing and upgrading the existing shell into livable space — insulating, finishing the walls/ceiling/floor, adding or upgrading windows and an entry door, and installing the kitchen, bathroom, plumbing, electrical, and HVAC needed for a dwelling. This is significantly less than constructing an entirely new detached building. By comparison, a detached new-build ADU requires a complete new structure (foundation, framing, roofing, siding, etc.), making it the most expensive type, while an attached addition falls in between, and basement/interior conversions are similarly economical (using existing space). Things to consider with a garage conversion: you lose the garage (covered parking/storage), and some jurisdictions require you to replace the lost parking elsewhere on the property (which adds cost) — though many areas have waived or reduced parking requirements for ADUs to encourage them; the existing garage slab and structure must be suitable (or be upgraded) to meet residential building codes (the slab may need work, the structure may need reinforcement, insulation, proper egress, ceiling height, moisture control, etc.); utilities must be extended/upgraded for a dwelling; and permits are required to legally convert it to living space. Detached garage conversions can function much like a detached ADU (private), while attached garage conversions connect to the house. Despite these considerations, garage conversions typically offer the lowest-cost path to an ADU and make good use of underused space. This calculator includes garage conversion as the most economical ADU type. If you have a suitable garage and don't mind losing the parking/storage (or can replace it), a conversion is often the most budget-friendly way to add an ADU. A contractor can assess whether your garage is a good candidate and what upgrades it needs to meet code.

Yes — ADUs generally add significant value to a property, through multiple avenues: increased property value, rental income potential, and added functional living space, which is a major reason ADUs have become so popular. Property value: adding an ADU typically increases a home's market value, as it adds livable square footage and a versatile, income-capable unit; buyers often pay a premium for properties with ADUs (for the flexibility, multigenerational living, or rental income), and in many markets an ADU can add substantial value — often a good portion of (and sometimes exceeding) its construction cost, especially in high-demand or high-cost areas where extra housing is valuable. The exact value-add depends on the local market, the ADU's quality and size, and demand. Rental income: a major financial benefit is the ability to rent the ADU (long-term or, where allowed, short-term) for ongoing income, which can offset the construction cost over time and the mortgage, and provides a return on the investment — this income potential is a key driver of ADU value, effectively making the property income-producing. Functional value: ADUs provide flexible space for multigenerational living (housing aging parents or adult children, keeping family close while independent — often at lower cost than senior care or separate housing), a home office, guest quarters, or other uses, which has real personal value even if not monetized. Considerations: the value/return depends on local regulations (areas that allow ADUs and especially rentals see more value), the market, the cost to build (high build costs can affect ROI), and quality; financing and the upfront cost are hurdles; and you take on landlord responsibilities if renting. Some areas' rules on renting (or short-term rentals) affect income potential. Overall, ADUs are widely regarded as a strong investment that adds property value, generates income, and provides flexible housing — particularly in markets with housing demand and ADU-friendly regulations. This calculator estimates the construction cost; weigh it against the rental income, value increase, and personal-use benefits for your situation and market. ADUs often deliver strong returns, but local market and rules matter. Research your area's ADU regulations and rental market to gauge the value for you.

ADUs are subject to local zoning and building regulations that vary significantly by jurisdiction, and navigating these rules (and obtaining permits) is a critical, sometimes complex part of building an ADU — though many areas have streamlined ADU rules to encourage them. Key regulations that commonly apply: Zoning/eligibility — whether ADUs are allowed on your property, and what types (detached, attached, conversion); some areas broadly permit ADUs (recent laws in many states/cities have legalized and eased them), while others restrict them. Size limits — maximum (and sometimes minimum) square footage for the ADU, often tied to the lot or primary home size. Setbacks — required distances from property lines (especially for detached units). Height limits — maximum height, particularly for detached ADUs. Parking — whether you must provide off-street parking for the ADU (many areas have reduced or eliminated this requirement to encourage ADUs, but some still require it). Owner-occupancy — some jurisdictions require the owner to live in the primary home or the ADU. Utilities — rules on connections, separate meters, and impact/connection fees for water/sewer. Building codes — the ADU must meet residential building codes (egress, ceiling height, fire safety, insulation, etc.), and you'll need building permits and inspections. Design standards — some areas require the ADU to match the main home or meet aesthetic rules. The permitting process involves submitting plans (often requiring an architect/designer and engineering), paying permit and impact fees (which can be substantial), and passing inspections. Because rules vary so much (and change as jurisdictions update ADU laws), it's essential to research your local regulations early (check with your city/county planning department) or work with a professional familiar with local ADU rules, as the regulations affect whether, what type, what size, and how you can build — and the associated costs (design, permits, fees). This calculator includes a design/engineering/permits add-on to account for these costs, but local fees vary widely. Start by confirming your area's ADU rules and permit requirements before designing. Many areas now have ADU-specific (sometimes streamlined or pre-approved) programs. A local designer/contractor can guide you through the process.

Connecting an ADU to utilities (water, sewer, electrical, and sometimes gas) is an important and potentially costly part of the project, and how it's handled depends on the ADU type, whether utilities are shared with the main house or separate, and local requirements. The basics: an ADU needs water supply, sewer/septic, electrical, and HVAC (and possibly gas), and these can either be extended from the existing home's systems (shared) or set up as separate services/meters for the ADU. Shared vs. separate: many ADUs tap into the existing home's water, sewer, and electrical systems (extending lines to the ADU), which is generally cheaper than installing separate utility services; however, the existing service may need upgrading to handle the added load (e.g., a larger electrical panel/service, or confirming the water/sewer capacity). Some owners or jurisdictions opt for (or require) separate meters for the ADU — separate electric, water, and/or gas meters — which is more expensive to install but allows separate billing (useful for rentals, so the tenant pays their own utilities) and clearer usage tracking. Detached ADUs and trenching: for a detached ADU, utility lines (water, sewer, electrical, gas) must be run from the main house or the street out to the unit, which requires trenching across the yard and connecting the lines — this trenching and the distance can add significant cost, more so the farther the unit is from the connections. Sewer/septic: the ADU connects to the existing sewer lateral or septic system; if on septic, the system must have adequate capacity (or be expanded), which can be a major cost. Capacity upgrades and fees: utility connection or impact fees may apply, and service upgrades (electrical panel, water line size) add cost. HVAC: the ADU typically needs its own heating/cooling (often a mini-split system) since it's a separate living space. Planning utilities early is important, as they affect both cost and feasibility. This calculator includes add-ons for separate meters/hookups, trenching utilities to a detached ADU, and a dedicated HVAC system to capture these costs. Your contractor and the utility providers will determine whether to share or separate services and what upgrades are needed. Utilities are a key cost driver, especially for detached ADUs requiring trenching and potential service upgrades.

Building an ADU typically takes several months, and the full process from planning to move-in often spans roughly 6 to 12+ months, with the construction itself usually a few months and the design/permitting phase adding significant time. Breaking it down: Design and planning (often 1-3+ months) — designing the ADU, creating construction plans (with an architect/designer and engineering), and finalizing the scope; this can take longer for custom designs. Permitting (often 1-6+ months, highly variable) — submitting plans to the local building department, plan review, and obtaining permits; this varies enormously by jurisdiction (some areas with streamlined or pre-approved ADU programs are faster, while others have long review times), and is often the most unpredictable part of the timeline. Construction (often 3-6+ months) — the actual building: for a detached new-build, this includes site work, foundation, framing, roofing, exterior, then interior systems (plumbing, electrical, HVAC), insulation, drywall, finishes, kitchen and bath installation, and final inspections; a garage or interior conversion is often faster (a couple to a few months) since the structure exists, while a detached new-build takes longer. Inspections throughout and final approval add time. Factors affecting the timeline include the ADU type (conversions are generally faster than detached new-builds), the size and complexity, the design and finish level, the permitting speed in your area (a big variable), site conditions and utility work (trenching for a detached unit), the contractor's schedule and crew, weather, and any delays (permit revisions, material/labor availability, change orders). Prefab/modular ADUs can shorten construction time (built in a factory and installed), though site prep and permitting still apply. To keep things moving, start the design and permitting early (since permitting is often the bottleneck), work with professionals experienced in local ADU processes, and plan for the full timeline. This calculator estimates the cost; the schedule depends mainly on the ADU type, the permitting timeline in your jurisdiction, and the construction scope. Plan for the better part of a year (or more) from concept to completion, especially for a detached new-build. Your contractor can provide a timeline based on your project and local permitting.